From man sudo
:
-s, --shell Run the shell specified by the SHELL environment variable if it is set or the shell specified by the invoking user's password database entry. If a command is specified, it is passed to the shell for execution via the shell's -c option. If no command is specified, an interactive shell is executed.
So -s
keeps your current shell (bash in this case), while omitting it uses the shell of the root user (sh). Myself, I prefer sudo -Es
to keep both my shell and environment variables.